Particulate minerals are useful in the treatment of sewage as they aid the clarification of waste water in, for example, conventional sedimentation processes, especially at the primary sedimentation stage. Thus, lime is well known as a coagulant in sewage sedimentation and in sludge conditioning prior to filtration. Lime is often used with the separate addition of other flocculants/coagulants, e.g. high molecular weight cationic polyacrylamides, ferric chloride, ferrous sulphate or alum.
This lime is normally used as a powder or as a dilute (say 10% solids) slurry prepared, for instance, by slaking on site. However, this requires plant to handle powdered lime or to slake quicklime; furthermore, powdered lime is an unpleasant material to work with. Moreover, the separate addition to the aqueous system of lime and other additives creates problems on site with handling, proportioning and metering.
Other particulate minerals, such as calcium carbonate are useful in sewage treatment, for example, as weighting agents. A mixture of hydrated lime and calcium carbonate can be particularly useful, as described in United Kingdom Patent Specification No. 2,095,226.
In papermaking, mineral particles such as calcium carbonate are used as fillers and pigments. The use of flocculated mineral particles is advantageous in that it allows higher than usual filler-loading levels to be achieved without loss of paper strength (see, for example, the discussion in U.K. Patent Specification No. 2,016,498). The flocculant may be added separately to the papermaking stock, before or after the filler is added as a slurry.
As an alternative to separate addition, both in sewage or other waste-water treatment and in papermaking, the flocculant may be added to a slurry of the mineral particles immediately before use. This requires special plant to ensure good mixing of the components and to ensure that the resultant suspension is conveyed immediately to the point of use. There are severe difficulties, however, in incorporating conventional flocculants into higher solids content slurries as they cause gross flocculation and thickening such that the resultant slurry does not flow readily and is not readily pumpable except with special equipment.
It would clearly be desirable to provide suspensions of mineral particles having a high solids content that are readily and economically transportable, especially such suspensions that contain the other components required to carry out the desired coagulation/flocculation process. Such suspensions could be manufactured with consistent quality and they would simplify on-site dosage and application, being a `one-pack` product. Thus, they could obviate or reduce the need for expensive plant on site to handle powders, liquids or slurries, to proportion and meter components added separately or to mix particle suspensions and a flocculant at high speed. Furthermore, the higher the solids content, the greater is the reduction in the costs entailed in transporting water.
However, to be practicable, such suspensions of high solids content would have to be simple to handle and apply, and be readily pumped/metered at controlled rates. Here, two problems must be overcome. Firstly, the viscosity characteristics of high solids slurries deteriorate rapidly as their solids content is increased. For example, for a lime slurry with no additives and having a solids content of 50% by weight, the following measurements have been made: a yield of 1335 dynes/cm.sup.2 and an apparent viscosity at 600 s.sup.-1 shear rate of 264 cP (measurements made on a Haake Rotovisco RV2 viscometer using cup MV St and bob MVI).
Some reduction in viscosity can be achieved by the incorporation of a small amount of a dispersing or surface-active agent such as a polyphosphate, a salt of a polyacrylic acid or a salt of polymethacrylic acid. The latter two are preferred in water/sewage treatment. The use of these materials alone, however, generally leads to the production of a stubborn sediment on settling.
Secondly, the addition of a flocculant to a mineral suspension normally causes heavy flocculation of the particles and results in a thick paste which is difficult to handle in processing plant, slurry tankers, and the like.